The novel existed first. And why couldn’t you imagine it? It’s a good story, not just a bunch of shark-popping-out shots.
There were some plot lines in the book that weren’t in the movie:
Ellen Brody and Hooper had an affair. Also, there was a connection between Mayor Vaughan and the Mafia, further explaining why he was downplaying the shark attacks and working so hard to keep the beaches open.